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.\" $Id: elf_update.3 3957 2022-03-12 14:11:52Z jkoshy $
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.Dd April 22, 2019
.Dt ELF_UPDATE 3
.Os
.Sh NAME
.Nm elf_update
.Nd update an ELF descriptor
.Sh LIBRARY
.Lb libelf
.Sh SYNOPSIS
.In libelf.h
.Ft off_t
.Fn elf_update "Elf *elf" "Elf_Cmd cmd"
.Sh DESCRIPTION
Function
.Fn elf_update
causes the library to recalculate the structure of an ELF
object and optionally write out the image of the object
to file.
.Pp
Argument
.Fa elf
should reference a valid ELF descriptor.
.Pp
Argument
.Fa cmd
can be one of the following values:
.Bl -tag -width "Dv ELF_C_WRITE"
.It Dv ELF_C_NULL
The library will recalculate structural information flagging
modified structures with the
.Dv ELF_F_DIRTY
flag, but will not write data to the underlying file image.
.It Dv ELF_C_WRITE
The library will recalculate structural information and will
also write the new image to the underlying file.
The ELF descriptor referenced by argument
.Fa elf
should permit the underlying ELF object to be written or updated
(see
.Xr elf_begin 3 ) .
.El
.Pp
All pointers to
.Vt Elf_Scn
and
.Vt Elf_Data
descriptors associated with descriptor
.Fa elf
should be considered invalid after a call to
.Fn elf_update .
.Ss Specifying Object Layout
The
.Lb libelf
supports two layout modes.
.Bl -tag -width indent
.It "Library Layout"
If the
.Dv ELF_F_LAYOUT
flag is not set on the ELF descriptor, the ELF library will lay out
the ELF object according to the following scheme:
.Bl -tag -compact -width "Section Data"
.It Em EHDR
The ELF executable header will be placed at the start of the object.
.It Em PHDR
If the ELF descriptor contains a program header table, it will be
placed after the Executable Header.
.It Em Section Data
ELF section data, if any, will be placed next, keeping each section's
alignment requirements in mind.
.It Em SHDR
The ELF section header table, if any, will be placed last.
.El
.It "Application Controlled Layout"
The application can take full control of the layout of the ELF object
by setting the
.Dv ELF_F_LAYOUT
flag on the ELF descriptor (see
.Xr elf_flagelf 3 ) .
In this case the library will lay out the ELF object using
application-supplied information as below:
.Pp
.Bl -tag -compact -width "Section Data"
.It Em EHDR
The ELF executable header will be placed at the start of the object.
.It Em PHDR
The ELF program header table, if any, it will be placed at the offset
specified in the
.Va e_phoff
field of the ELF executable header.
.It Em Section Data
The data for each ELF section will be placed at the offset specified
by the
.Va sh_offset
field of the section's header.
The size of the section will be taken from the
.Va sh_size
field of the section header.
.It Em SHDR
The ELF section header table, if any, will be placed at the offset
specified by the
.Va e_shoff
field of the executable header.
.El
.El
.Pp
Gaps in the coverage of the file's contents will be set to the fill value
specified by
.Xr elf_fill 3 .
.Ss Application Supplied Information
The application needs to set the following fields in the data
structures associated with the ELF descriptor prior to calling
.Fn elf_update .
.Bl -tag -width indent
.It "Executable Header"
The fields of the ELF executable header that need to be set by the
application are:
.Pp
.Bl -tag -width "e_ident[EI_OSABI]" -compact
.It Va e_entry
To be set to the desired entry address for executables.
.It Va e_flags
To be set to the desired processor specific flags.
.It Va "e_ident[EI_DATA]"
Must be set to one of
.Dv ELFDATA2LSB
or
.Dv ELFDATA2MSB .
.It Va "e_ident[EI_OSABI]"
To be set to the OS ABI desired.
For example, for
.Fx
executables, this field should be set to
.Dv ELFOSABI_FREEBSD .
.It Va e_machine
To be set to the desired machine architecture, one of the
.Dv EM_*
values in the header file
.In elfdefinitions.h .
.It Va e_phoff
If the application is managing the object's layout, it must
set this field to the file offset of the ELF program header table.
.It Va e_shoff
If the application is managing the object's layout, it must
set this field to the file offset of the ELF section header table.
.It Va e_shstrndx
To be set to the index of the string table containing
section names.
.It Va e_type
To be set to the type of the ELF object, one of the
.Dv ET_*
values in the header file
.In elfdefinitions.h .
.It Va e_version
To be set to the desired version of the ELF object.
.El
.It "Program Header"
All fields of the entries in the program header table need to be
set by the application.
.It "Section Header"
The fields of ELF section headers that need to be set by the
application are:
.Pp
.Bl -tag -width "sh_addralign" -compact
.It Va sh_addr
To be set to the memory address where the section should reside.
.It Va sh_addralign
If the application is managing the file layout, it must set this
field to the desired alignment for the section's contents.
This value must be a power of two and must be at least as large as the
largest alignment needed by any
.Vt Elf_Data
descriptor associated with the section.
.It Va sh_entsize
To be set to the size of each entry, for sections containing fixed size
elements, or set to zero for sections without fixed size elements.
If the application is not managing file layout, it may leave this
field as zero for those sections whose types are known to the library.
.It Va sh_flags
To be set to the desired section flags.
.It Va sh_info
To be set as described in
.Xr elf 5 .
.It Va sh_link
To be set as described in
.Xr elf 5 .
.It Va sh_name
To be set to the index of the section's name in the string table
containing section names.
.It Va sh_offset
If the application is managing the file layout, it must set this
field to the file offset of the section's contents.
.It Va sh_size
If the application is managing the file layout, it must set this
field to the file size of the section's contents.
.It Va sh_type
To be set to the type of the section.
.El
.It "Section Data"
The
.Vt Elf_Data
descriptors associated with each section specify its contents
(see
.Xr elf_getdata 3 ) .
While all the fields in these descriptors are under application
control, the following fields influence object layout:
.Bl -tag -width "Va d_align" -compact
.It Va d_align
To be set to the desired alignment, within the containing section, of
the descriptor's data.
.It Va d_off
If the application is managing object layout, it must set this field
to the file offset, within the section, at which the descriptor's data
should be placed.
.It Va d_size
To be set to the size in bytes of the memory representation of the
descriptor's data.
.El
.El
.Sh RETURN VALUES
Function
.Fn elf_update
returns the total size of the file image if successful, or -1 if an
error occurred.
.Sh ERRORS
This function may fail with the following errors:
.Bl -tag -width "[ELF_E_RESOURCE]"
.It Bq Er ELF_E_ARGUMENT
Argument
.Fa elf
was null.
.It Bq Er ELF_E_ARGUMENT
Argument
.Fa cmd
was not recognized.
.It Bq Er ELF_E_ARGUMENT
The argument
.Fa elf
was not a descriptor for an ELF object.
.It Bq Er ELF_E_CLASS
The
.Va e_ident[EI_CLASS]
field of the executable header of argument
.Fa elf
did not match the class of the file.
.It Bq Er ELF_E_DATA
An
.Vt Elf_Data
descriptor contained in argument
.Fa elf
specified an unsupported type.
.It Bq Er ELF_E_DATA
An
.Vt Elf_Data
descriptor specified an alignment that was zero or was not a power of
two.
.It Bq Er ELF_E_HEADER
The ELF header in argument
.Fa elf
requested a different byte order from the byte order already
associated with the file.
.It Bq Er ELF_E_IO
An I/O error was encountered.
.It Bq Er ELF_E_LAYOUT
An
.Vt Elf_Data
descriptor contained in argument
.Fa elf
specified an alignment incompatible with its containing section.
.It Bq Er ELF_E_LAYOUT
Argument
.Fa elf
contained section descriptors that overlapped in extent.
.It Bq Er ELF_E_LAYOUT
Argument
.Fa elf
contained section descriptors that were incorrectly aligned or were
too small for their data.
.It Bq Er ELF_E_LAYOUT
The flag
.Dv ELF_F_LAYOUT
was set on the Elf descriptor and the executable header overlapped
with the program header table.
.It Bq Er ELF_E_LAYOUT
The flag
.Dv ELF_F_LAYOUT
was set on the Elf descriptor and the program header table was placed
at a misaligned file offset.
.It Bq Er ELF_E_LAYOUT
The flag
.Dv ELF_F_LAYOUT
was set on the Elf descriptor and the section header table overlapped
an extent mapped by a section descriptor.
.It Bq Er ELF_E_LAYOUT
The
.Dv ELF_F_LAYOUT
flag was set on the Elf descriptor, and the
.Va d_offset
field in an
.Vt Elf_Data
descriptor contained a value that was not a multiple of the
descriptor's specified alignment.
.It Bq Er ELF_E_MODE
An
.Dv ELF_C_WRITE
operation was requested with an ELF descriptor that was not opened for
writing or updating.
.It Bq Er ELF_E_SECTION
Argument
.Fa elf
contained a section with an unrecognized type.
.It Bq Er ELF_E_SECTION
The section header at index
.Dv SHN_UNDEF
had an illegal section type.
.It Bq Er ELF_E_SEQUENCE
An
.Dv ELF_C_WRITE
operation was requested after a prior call to
.Fn elf_cntl elf ELF_C_FDDONE
disassociated the ELF descriptor
.Fa elf
from its underlying file.
.It Bq Er ELF_E_UNIMPL
Argument
.Fa elf
contained a section with an unsupported ELF type.
.It Bq Er ELF_E_VERSION
Argument
.Fa elf
had an unsupported version or contained an
.Vt Elf_Data
descriptor with an unsupported version.
.El
.Sh SEE ALSO
.Xr elf 3 ,
.Xr elf32_getehdr 3 ,
.Xr elf32_getphdr 3 ,
.Xr elf32_newehdr 3 ,
.Xr elf32_newphdr 3 ,
.Xr elf64_getehdr 3 ,
.Xr elf64_getphdr 3 ,
.Xr elf64_newehdr 3 ,
.Xr elf64_newphdr 3 ,
.Xr elf_begin 3 ,
.Xr elf_cntl 3 ,
.Xr elf_fill 3 ,
.Xr elf_flagehdr 3 ,
.Xr elf_flagelf 3 ,
.Xr elf_getdata 3 ,
.Xr elf_getscn 3 ,
.Xr elf_newdata 3 ,
.Xr elf_newscn 3 ,
.Xr elf_rawdata 3 ,
.Xr gelf 3 ,
.Xr gelf_newehdr 3 ,
.Xr gelf_newphdr 3 ,
.Xr elf 5
